Anchoring arrangement for floating wind turbine installations

ABSTRACT

The anchoring device for a floating wind turbine installation, such wind turbine installation comprising a floating cell ( 7 ), a tower ( 8 ) arranged over the floating cell, a generator ( 9 ) mounted on the tower which is rotatable in relation to wind direction and fitted with a wind rotor ( 10 ), and an anchor line arrangement ( 6 ) connected to anchors or anchoring points on the sea bed. 
     The individual anchor lines ( 11 ) are each, at a certain distance from the floating cell ( 7 ) at a fixing point ( 5 ) on the individual anchor line, connected with double lines ( 2, 3 ) slanting outwards and connected to the floating cell ( 7 ) in a delta-shaped arrangement.

The present invention relates to an anchoring arrangement for a floatingwind turbine installation, such wind turbine installation comprising afloating cell, a tower arranged over the floating cell, a generatormounted on the tower which is rotatable in relation to wind directionand fitted with a wind rotor, and an anchor line arrangement connectedto anchors on the sea bed.

The advantage of using floating wind turbines is that this allows almostunlimited access to installation areas, since relatively deep sea areascan be used.

Normally, wind turbines are arranged such that the rotor faces the windwith the tower positioned downstream of the wind direction. This is toavoid the wind flow being disturbed before it passes through the rotor,which could otherwise lead to loss of energy and disruptive vibrationsand impulses in the turbine.

To control this, active direction correction of the turbine around thetower's vertical axis is required as the wind changes direction. This isnormally achieved by arranging a rotating bearing rim with a ring andpinion solution between the top of the tower and the nacelle.

Yawing is performed by a system which registers the wind direction andautomatically drives the pinion by a motor to make the rotor turn intothe direction of the wind.

This works well when the tower is on a fixed foundation, as in landinstallations and offshore installations in shallow waters.

When the tower is mounted on a floating support, it is necessary toensure sufficient resistance to rotation round the vertical axis, sothat active rotation of the nacelle is performed rather than the wholeplant rotating too far when the wind turbine is subjected to obliquewind loads.

Resistance to rotation is provided by anchor lines which arepre-tensioned by a specific force. When anchor lines are fixed directlyto a slim cylindrical construction, then, as described below, this giveslow resistance to rotation as the anchor line is connected near therotation axis. Resistance to rotation occurs when the tower is rotatedfrom its equilibrium position and a righting arm occurs as a function ofangle (

) and radius (r) from the rotation axis to the line fixing point. Therighting arm (a) is in this case:

a=sin(

)×r

The righting force corresponds to the normal component F_(n) on therotation axis of the pre-tension on the line, and the righting momentwill then become:

M _(r) =F _(n) ×a=F _(n)×sin(

)×r

The resistance moment against rotation is thus, as shown above, a sinefunction with a maximum at 90° (see below). At small angles of rotationthe rotation resistance will act like a linear rotation spring.

From U.S. Pat. No. 3,082,608 an anchored platform of triangular designhas previously been disclosed. From each corner, preferably at a 20°angle, two chains or cables are extended which are joined to heavyweights arranged on the sea bed, while from each weight furtheranchoring lines extend to heavy anchors sited further away from theplatform. The purpose of this solution is primarily an anchoringsolution designed to eliminate platform motion caused by waves. Thesolution will provide resistance to rotation but will be unsuitable foranchoring a slim cylindrical floating wind turbine, because itpresupposes that there is a great distance between the three fixingpoints for the anchors lines. The known solution is based on a tautanchoring system, generating very large dynamic forces in the anchorlines. In addition the anchoring solution is heavy and complex, which inturn necessitates high costs of manufacture and installation.

The present invention provides a solution for anchoring a floating windturbine installation by which it is possible to increase significantlythe initial resistance to rotation round the vertical axis. It furtherprovides a solution which is extremely simple and can be used foranchoring wind turbine installations in very deep water.

The invention is characterised in that the individual anchor lines, at acertain distance from the floating cell and at a fixing point on theindividual anchor line, are each connected to at least two separatelines which slant outwards and are fixed to the floating cell in adelta-shaped arrangement, as indicated in the attached independent claim1.

The dependent claims 2-6 define advantageous features of the invention.

The invention will be further described by means of an example and withreference to the attached figures in which:

FIG. 1 shows a perspective sketch of a floating wind turbine with ananchoring arrangement according to the present invention,

FIG. 2 shows a skeleton sketch of the anchoring arrangement in relationto the invention shown in FIG. 1 and

FIG. 3 shows a diagram in which rotation resistance (rotation moment) iscalculated on the basis of the rotation angle for a conventional anchorsystem compared with the present invention.

As mentioned, FIG. 1 shows a perspective sketch of a floating windturbine installation 1 with an anchoring arrangement 6 according to theinvention. The wind turbine includes, apart from anchoring lines 11, apreferably circular elongated floating cell 7, a tower 8 mounted on thefloating cell 7, and on top of the tower a generator 9 which can berotated in relation to wind direction, bearing a wind rotor 10. Theelongated shape has been selected from a desire to achieve lowdisplacement with good stability and thus minimal effect from wind andwaves. Weights 12 can further advantageously be arranged on the anchorlines to create the necessary tension in these.

As is further evident from the anchor line arrangement 6 shown in FIG.2, three anchor lines 11 have been used at intervals of 120°. Theindividual anchor lines 11 are each fixed at one end to anchors oranchoring points on the sea bed (not shown), and at the other end, at acertain distance from the floating cell 7 at a fixing point 5, they areconnected to two lines 2 and 3 which slant outwards and are fixed tofloating cell 7 at paired jointly arranged brackets. 4. Each of theanchor lines 11 forms with these a delta-shaped pattern or Y-shapedbifurcation at/towards the fixing point on floating cell 7. In thiscontext it must be noted that even though the example uses one line 2and one line 3, each extended at the same angle towards their respectivefixing brackets 4 on the floating cell, two or more lines 2 and two ormore lines 3 may be used, each extending at different angles towardsvarious fixing brackets on the floating cell.

The length of lines 11 is relatively long, depending on the depth of thesea bed where the wind turbine is located, and the pre-tensioning in theindividual anchor lines may be of the order of 1000 kN. The lines' angleto the horizontal plane is approx. 30-70° and the length of the lines 2,3, depending on the dimensions of the wind turbine installation and ofthe floating cell as a whole, may be of the order of 50 m.

With these suggested values, calculation shows that the arrangementaccording to the invention is of the order of 9 times more resistant torotation than it would have been with a conventional solution in whichthe anchor lines are fixed directly to the floating cell, without thelines being arranged in a delta shape.

The characteristics of rotational stiffness for a conventional solutionand for the delta line solution according to the invention are shown inFIG. 3.

As mentioned, FIG. 3 shows a diagram in which rotation resistance(rotation moment) is calculated on the basis of the rotation angle for aconventional anchor system compared with the present invention. Duringrotation round the vertical rotation axis, the pre-tensioning load fromthe anchor line will gradually increase in one split line, while load onthe other is correspondingly relieved. When the rotation angle reaches acertain magnitude, the relieved line will become slack. The angle ofrotation at which slack occurs will depend on the length of the deltalines, or the distance between the split point and the vertical rotationaxis. For small angles, before slack occurs in one line, the arrangementwill function as if the fixing point on the buoy has been moved out tothe fixing point on the anchor line. This will give a large arm R, suchthat the righting moment will be:

Mr=Fn×sin(

)×R

When the angle of rotation reaches a critical value (

) so that there is slack on one line, the moment increase will be smalluntil the maximum moment is reached. The critical angle for slack in oneof the delta lines in the example calculated here, as shown in FIG. 3,is of the order of 6°. The curve here changes direction at a momentupwards of 14000 kNm.

With the increased initial resistance to rotation achieved with thisarrangement, active direction control of the turbine can be effectedwith an acceptable response angle in the tower.

For a conventional anchor system, it can further be seen from the figurethat the resistance to rotation increases the slack until a maximumrotation resistance is reached close to an angle of 90°.

The invention as defined in the claims is not limited to the embodimentshown in the figures and described in the foregoing, so that instead ofthree anchor lines, four or more anchor lines 11 may be used, each withcorresponding lines 2, 3 arranged in a delta-shaped pattern. Use ofthree anchor lines at intervals of 120° is however seen as representingthe simplest and cheapest solution.

1-6. (canceled)
 7. Anchoring arrangement for a floating wind turbineinstallation, such wind turbine installation comprising a floating cell(7), a tower (8) arranged over the floating cell, a generator (9)mounted on the tower which is rotatable in relation to wind directionand fitted with a wind rotor (10), and an anchor line arrangement (6)connected to anchors or anchor points on the sea bed wherein theindividual anchor lines (11) are each, at a certain distance from thefloating cell (7) at a fixing point (5) on the individual anchor line,connected with at least two lines (2, 3), each slanting outwards andbeing connected to the floating cell (7) in a delta-shaped arrangement.8. Anchoring arrangement according to claim 7, wherein the anchor linearrangement (6) comprises three anchor lines (11) arranged symmetricallyat intervals of 120°.
 9. Anchoring arrangement according to claim 7,wherein lines (2, 3) in the delta-shaped arrangement are arranged at amutual angle of between 20 and 60°.
 10. Anchoring arrangement accordingto claim 7, wherein lines (2, 3) in the delta-shaped arrangement arearranged at an angle to the horizontal plane of between 30 and 70°. 11.Anchoring arrangement according to claim 7, wherein lines (2, 3) in thedelta-shaped arrangement are fixed to the floating cell (7) at themaximum possible distance for the fixing points on the floating cell.12. Anchoring arrangement according to claim 7, wherein lines (2, 3) inthe delta-shaped arrangement are fixed in pairs to joint brackets (4) onthe floating cell (7).